Due to higher cost of base station generally, the construction of numerous base station will lead to higher net laying cost in the development of 3G technology (3rd generation mobile communication). For the purpose of reducing the cost, relay is introduced into the cellular communication system gradually to extend the coverage area of base station signal. FIG. 1a is overall LTE-A network architecture with deployed Relay Node (RN), which is wirelessly connected to the core network via the donor cell under a Donor Evolved Node B (DeNB). There is no direct wired interface between the RN and core network and each RN can manage one or more cells. In this architecture, the interface between User Equipment (UE) and RN is called Uu interface, while that between RN and DeNB is Un interface. One DeNB can have several RNs connected, while one RN can only connected to one DeNB.
FIG. 1b is the structure chart of LTE-A system after introduction of RN. S1 interface has one end connected to the node eNB in E-UTRAN (Evolved UTRAN) and the other connected to the node MME/S-GW (Mobility Management Entity/Serving Gateway) in EPC (Evolved Packet Core), or one end connected to eNB and the other, to RN. S1 interface provides the access to radio resource in radio access network, including control plane function and user plane function.
Control plane interface of S1 (S1-MME) provides the application protocol between eNB and MME, and the signaling bearing function used for transferring application protocol message, while the user plane interface of S1 (S1-U) provides the user plane data transmission function between eNB and S-GW. S1 interface mainly includes the following functions: E-RAB (E-UTRAN Radio Access Bearer) management function; mobility management function; paging function; NAS (Non-Access Stratum) signaling transmission function; LPPa (LTE Positioning Protocol A) signaling transmission function; S1 interface management function; network sharing function, roaming and regional lockout support function; NAS node selection function; initial context setup function; UE context modification function; MME load balancing function; position report function; PWS (Public Warning System) message transmission function; overload function; RAN (Radio Access Network) message management function; configuration transmission function; S1 CDMA2000 tunnelling function, etc.
X2 interface is between nodes eNB or eNB and RN in E-UTRAN, and X2 interface of many-to-many connections exists between eNBs, so there may be the possibility that all eNBs within a certain area have X2 connection. The existence of X2 interface is mainly to support the mobility management function of UE under connection status; besides, it is also functioned as load management function, cell interference coordination function, general X2 management and error control function, etc.
As shown in FIG. 2, the start-up procedure of RN can be described as: RRC (Radio Resource Control) connection established between RN and DeNB; attach request sent to MME by RN; RN signing data acquired by MME from HSS (Home Subscriber Server), and verification on RN; in case the verification is qualified, MME establishes default bearing for RN in S-GW/P-GW and send initial context setup request message to DeNB for setting up RN context in DeNB; then DeNB sends RRC connection reconfiguration message to RN and carry the attach acceptance message sent by MME to RN which returns to RRC connection reconfiguration for finishing confirmation. In such way, RN establishes the basic IP connection. Later, O&M (Operation and Maintenance) downloads the node configuration information to RN to configure RN. Then RN establishes necessary S1 interface and X2 interface to enable normal work like base station.
The process of wiring UE into RN for attachment is as shown in FIG. 3: UE establishes RRC connection with RN and send attach request message to MME. RN carries such NAS message in initial UE message and send it to DeNB. After receiving, DeNB will modifies the eNB UE S1-AP ID1 therein into eNB UE S1-AP ID2 distributed for UE and send the message to proper MME in accordance with GUMMEI (Globally Unique MME Identifier) information indication or MME selection function. After receiving initial UE message, MME distributes one MME UE S1-AP ID3 for the UE and save it with the received eNB UE S1-AP ID2 correspondingly. MME UE S1-AP ID3 only identifies one UE in one MME, then MME distinguishes UE based on the MME UE S1-AP ID3 in received S1-AP message. MME sends initial context setup request message to the affiliated DeNB of RN, and DeNB replaces the received MME UE S1-AP ID3 by its own distributed MME UE S1-AP ID4, then saves it with eNB UE S1-AP ID2 distributed for UE and eNB UE S1-AP ID1 for UE by RN correspondingly. Later, DeNB transfers the initial context setup request message to RN for setting up UE context in RN, then RN saves the eNB UE S1-AP ID1 distributed for UE together with the received MME UE S1-AP ID4 correspondingly. After that, RN send initial context setup response message to MME for confirmation. During UE attach, correspondence of each node saving is given in Table 1. In follow-up process, through the mapping relationship established in UE attach, DeNB can find MME message (GUMMEI) for R-UE (UE under relay) connection according to UE AP-ID information. GUMMEI information refers to globally unique MME identifier, comprising PLMN (Public Land Mobile Network) ID, MME Group ID and MME code, with each GUMMEI corresponding to unique MME. Therein, correspondence of each node saving is as shown in Table 1
TABLE 1RNDeNBMMEeNB UE S1-AP ID1eNB UE S1-AP ID1(RN distribution)(RN distribution)eNB UE S1-AP ID2eNB UE S1-AP ID2(DeNB distribution)(DeNB distribution)MME UE S1-AP ID3MME UE S1-AP ID3(MME distribution)(MME distribution)MME UE S1-AP ID4MME UE S1-AP ID4(DeNB distribution)(DeNB distribution)GUMMEI
It can be learnt from the process mentioned above, DeNB is functioned by S1/X2 AP agency, only one S1 interface and X2 interface respectively exist between RN and DeNB, DeNB is connected with many MME/S-GWs, DeNB is in charge of MME control node selection function of R-UE, and RN is not informed of the MME information connected to R-UE. However, RN needs to acquire such information for judging handover type and MME correct addressing, hence a scheme for enabling RN to acquire the MME information connected to R-UE is required.